SQL Server 2008 R2 : Modifying Tables - Using T-SQL to Modify Tables, Using Object Explorer and the Table Designer to Modify Tables, Using Database Diagrams to Modify Tables

Regardless of the method you use, you must always exercise caution when modifying tables, particularly in a production environment. Table relationships and the impact to data that may already exist in a table are key considerations in modifying a table. A visual tool such as a database diagram can assist you in determining the impact to related tables and can be used to generate the T-SQL script. The following section looks at the underlying T-SQL that can be used to modify a table, and then we delve into the visual tools that can simplify your life and generate some of the T-SQL for you.

Using T-SQL to Modify Tables

You can modify tables in many different ways, including making changes to the columns, constraints, and indexes associated with a table. Some of the changes have a bigger impact on the database than others. Some modifications require that the modified table be dropped and re-created to effect the change. Fortunately, you can use the T-SQL ALTER TABLE statement to mitigate the database impact and help streamline many of the most common modifications. You can make the following types of changes by using the ALTER TABLE statement:

• Change a column property, such as a data type or NULL property.

• Add new columns or drop existing columns.

• Add or drop constraints.

• Enable or disable CHECK and FOREIGN KEY constraints.

• Enable or disable triggers.

• Reassign partitions.

• Alter an index associated with a constraint.

The following sections discuss a few examples of these types of changes to familiarize you with the ALTER TABLE command. 

Changing a Column Property

You can use the ALTER COLUMN clause of the ALTER TABLE command to modify column properties, including the NULL property or the data type of a column. Listing 1 shows an example of changing the data type of a column.

Listing 1. Changing the Data Type of a Column by Using ALTER TABLE

alter table titles
  alter column notes varchar(400) null

You must be aware of several restrictions when you modify the data type of a column. The following rules apply when altering columns:

• You cannot modify a text, image, ntext, or timestamp column.

• The column cannot be the ROWGUIDCOL for the table.

• The column cannot be a computed column or be referenced by a computed column.

• The column cannot be a replicated column.

• If the column is used in an index, the column length can only be increased in size. In addition, it must be of varchar, nvarchar, or varbinary data type, and the data type cannot change.

• If statistics have been generated using CREATE STATISTICS, the statistics must first be dropped before the column can be altered.

• The column cannot have a PRIMARY KEY or FOREIGN KEY constraint or be used in a CHECK or UNIQUE constraint. The exception is that a column with a CHECK or UNIQUE constraint, if defined as variable length, can have the length altered.

• A column with a default defined for it can have only the length, nullability, or precision and scale altered.

• If a column has a schema-bound view defined on it, the same rules that apply to columns with indexes apply.

Adding and Dropping Columns

You add columns to a table by using the ADD COLUMN clause. Listing 2 illustrates the addition of a new column named ISBN to the titles table.

Listing 2. Adding a Column by Using ALTER TABLE

ALTER TABLE titles
  add ISBN int null

When you use the ALTER TABLE statement to add a column, the new column is added at the end of the table. In most cases, this location is acceptable. The location of the column in the table generally has no bearing on the use of the table. There are, however, situations in which it is desired to have the new column added in the middle of the table. The ALTER TABLE statement does not work for this situation. To add a column in the middle of the table, you need to create a new version of the table with a different name and the columns in the desired order, copy the data from the old table, drop the old table, and rename the new table with the old table name. Alternatively, you can also accomplish this by using SSMS, as described in the following section.

There are also some issues you need to consider with regard to the null option specified for a new column. In the case of a column that allows nulls, there is no real issue: SQL Server adds the column and allows a NULL value for all rows. If NOT NULL is specified, however, the column must be an identity column or have a default specified. Note that even if a default is specified, if the column allows nulls, the column is not populated with the default if no value is provided for the column. You use the WITH VALUES clause as part of the default specification to override this and populate the column with the default.

With some restrictions, columns can also be dropped from a table. Listing 3 shows the syntax for dropping a column. You can specify to drop multiple columns, separated by commas.

Listing 3. Dropping a Column by Using ALTER TABLE

alter table titles
  drop column ISBN

The following columns cannot be dropped:

• A column in a schema-bound view

• An indexed column

• A replicated column

• A column used in a CHECK, FOREIGN KEY, UNIQUE, or PRIMARY KEY constraints

• A column associated with a default or bound to a default object

• A column bound to a rule

As mentioned earlier, the ALTER TABLE statement is not the only T-SQL statement you can use to modify tables. You accomplish some table changes by using T-SQL that drops and re-creates the tables that are being modified. The following sections look at some examples of these types of changes.

Using Object Explorer and the Table Designer to Modify Tables

The Object Explorer in SSMS is your window into the various tables available for modification in a database. You expand the Tables node in the Object Explorer tree and right-click the table you would like to modify. Then you select the Design option, and a Table Designer window appears, showing all the table columns. In addition, a Table Designer menu option appears at the top of the SSMS window. The Table Designer menu includes many options, including Insert Columns, Delete Columns, and Remove Primary Key. The full list of available options is shown in Figure 1. A Table Designer window for the BigPubs2008.Authors table is shown as the active tab on the right side of Figure 1.

Figure 1. The Table Designer.

To illustrate the power of the Table Designer, let’s add a new column to the authors table. You can add the column to the middle of the table, just prior to the address column. You do this by highlighting the entire address row in the Table Designer grid and then selecting Table Designer, Insert Column. A new data entry row is added to the Table Designer grid, where you can specify the name of the new column, the data type, and a null option. For this example, you can name the new column Gender, with a data type of char(1) and the setting ALLOW NULLS. Figure 2 shows the Table Designer grid with the newly added Gender column highlighted. In addition, the figure shows the Table Designer menu options with the newly enabled Generate Change Script option selected.

Figure 2. Inserting a column in Table Designer.

You do not need to use the Generate Change Script option for changes you make in the Table Designer. You can close the Table Designer tab where you made your changes, and the Table Designer makes the changes for you behind the scenes. Sometimes, though, you might want to script the changes and see exactly what is going to happen to the database. Clicking the Script button is also the preferred method for deploying changes to other environments. You can save a script in a change repository and execute it in your target environments. This approach ensures that you have a repeatable, well-documented means for making table changes.

Listing 4 shows the contents of a script that would be generated based on the new Gender column you added to the authors table. For the sake of space, some of the initial script options and the triggers associated with the authors table have been removed from the script. The important point to note is how extensive this script is. A new temporary authors table is created, and it includes the new column; the data from the authors table is copied into the temporary table; and then the table is renamed. In addition, the script must manage the constraints, indexes, and other objects associated with the authors table. The good news is that Table Designer does most of the work for you.

Listing 4. Changing Script Generated from the Table Designer

ALTER TABLE dbo.authors
   DROP CONSTRAINT DF__authors__phone__04C4C0F4
GO
CREATE TABLE dbo.Tmp_authors
   (
   au_id dbo.id NOT NULL,
   au_lname varchar(40) NOT NULL,
   au_fname varchar(20) NOT NULL,
   phone char(12) NOT NULL,
   Gender char(1) NULL,
   address varchar(40) NULL,
   city varchar(20) NULL,
   state char(2) NULL,
   zip char(5) NULL,
   contract bit NOT NULL
   )  ON [PRIMARY]
GO
ALTER TABLE dbo.Tmp_authors ADD CONSTRAINT
   DF__authors__phone__04C4C0F4 DEFAULT ('UNKNOWN') FOR phone
GO
IF EXISTS(SELECT * FROM dbo.authors)
    EXEC('INSERT INTO dbo.Tmp_authors (au_id, au_lname,
      au_fname, phone, address, city, state, zip, contract)
      SELECT au_id, au_lname, au_fname, phone, address,
         city, state, zip, contract
      FROM dbo.authors WITH (HOLDLOCK TABLOCKX)')
GO
ALTER TABLE dbo.titleauthor
   DROP CONSTRAINT FK__titleauth__au_id__14070484
GO
DROP TABLE dbo.authors
GO
EXECUTE sp_rename N'dbo.Tmp_authors', N'authors', 'OBJECT'
GO
ALTER TABLE dbo.authors ADD CONSTRAINT
   UPKCL_auidind PRIMARY KEY CLUSTERED
   (
   au_id
   ) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,
      ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
CREATE NONCLUSTERED INDEX aunmind ON dbo.authors
   (
   au_lname,
   au_fname
   ) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,
      ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = OFF) ON [PRIMARY]
GO
ALTER TABLE dbo.authors WITH NOCHECK ADD CONSTRAINT
   CK__authors__au_id__03D09CBB
      CHECK (([au_id] like
      '[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]'))
GO
ALTER TABLE dbo.authors WITH NOCHECK ADD CONSTRAINT
   CK__authors__zip__05B8E52D
      CHECK (([zip] like '[0-9][0-9][0-9][0-9][0-9]'))
GO


					  

You will find that you can make most of the changes you want to make by using the Table Designer. To make other changes, you can use the same approach you just used to add a column. This approach involves making the changes via the Table Designer menu options and then using the option to script the change. This is a great way to evaluate the impact of your changes and to save those changes for later execution.

Using Database Diagrams to Modify Tables

Database diagrams offer an excellent visual view of your database tables that you can also use to modify tables. You do this by adding the table you want to modify to a new or existing database diagram. Oftentimes, it is best to also add all the related tables to the diagram as well. You can easily do this by right-clicking the table and choosing the Add Related Tables option.

With a database diagram, you have the same options that you have with the Table Designer, plus you have diagramming options. Both the Table Designer and Database Diagrams menus are shown when a database diagram is in focus. These menus disappear if you change the tabbed window to a Database Engine query window, so remember that you must select the diagram window to be able to display the menu options.

Figure 3 shows a database diagram for the HumanResouces.Department table, along with its related table. The Table Designer menu is selected to show that it is available when you work with a database diagram. You must have one of the tables selected to enable all the menu options. In Figure 3, the Department table has been highlighted, and a new ModifiedUser column has been added to the end of the table. Figure 3 also shows that the Database Diagram menu is available for selection. This menu includes options to add tables to the diagram and manipulate the tables within the diagram.

Figure 3. Modifying tables by using a database diagram.

The same scripting options are available with a database diagram as are available in the Table Designer. You can make your changes from within the diagram and then choose the Generate Change Script menu option. Listing 5 shows the change script generated based on the addition of the ModifiedUser column to the end of the Department table. As expected, this change is accomplished with an ALTER TABLE statement.

Listing 5. The Change Script Generated from a Database Diagram

ALTER TABLE HumanResources.Department ADD
    ModifiedUser varchar(20) NULL
GO

The use of the ALTER TABLE statement in this listing brings us full circle, back to our initial method for making table modifications. Using all the tools discussed in this section together will usually give you the best results.